Author Topic: HVAC intake ductwork freezing when it's cold (and sweating on drywall)  (Read 11454 times)

Guses

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I am in the process of finishing my basement and I recently installed drywall ceilings in the basement.

There are some insulated HVAC vents for the air exchanger and furnace cold air intake that spawn over the bathroom. I have made a 2x4 box for the ducts to go in under the joists so that I could drywall over them without dropping the entire ceiling 8-10 inches.

I noticed that there was some ice forming on the ceiling at the point where the ducts came into the house. In fact, after removing a piece of the ceiling, I saw that there was some ice on the ducts as well.

We had some -30 F weather and it routinely goes to -40 sometimes. I am looking for ideas on how to fix this.

My two main hypothesis are that either:

1)The intake vents are leaky(they are shitty builder grade vents) and they allow cold air into the wall or snow during the storms.

2)The very cold air caused the insulated ducts to drop below dew point in the ceiling and ice formed on it and sweated on the ceiling.

I am able to "easily" access about 10 feet of ductwork from the point it enters the house as the joints are not completely done yet and I could remove portions of the ceiling. The rest of the ductwork leading to the HVAC equipment is not accessible.

My thinking was that by replacing the intake with higher quality ones and then insulating the crap out of the accessible ducts (using bat insulation in the 2x4 box), I should eliminate any sweating and ice build up or at least reduce it enough that it won't be a problem.

I also read that I could add a 200-300 watt heater (to be run when very cold) on the intake vents that could help prevent ice.

Thoughts? Suggestions?



 

jba302

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Re: HVAC intake ductwork freezing when it's cold (and sweating on drywall)
« Reply #1 on: January 14, 2015, 03:32:20 PM »
Insulation plus a vapor barrier will fix it.

guitar_stitch

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Re: HVAC intake ductwork freezing when it's cold (and sweating on drywall)
« Reply #2 on: January 16, 2015, 02:06:37 PM »
Shitty R-Value on the ductwork and high humidity?

Guses

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Re: HVAC intake ductwork freezing when it's cold (and sweating on drywall)
« Reply #3 on: January 17, 2015, 05:50:50 AM »
Shitty R value? Probably.

High humidity? Not that much RH is around 45-50.


I added a picture of the set up. Would sliding a piece of rigid r4 foam under the wood support fix the issue? It appears to be freezing and leaking mostly where it touches the wood cross members.




Guses

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Re: HVAC intake ductwork freezing when it's cold (and sweating on drywall)
« Reply #4 on: January 21, 2015, 03:04:48 PM »
I am bumbing this in case someone else has input.

I tried putting rigid insulation underneath the ducts, but it's a really tight fit and I am compressing the ducts even more. Would that be counterproductive to my issue?

 

Greg

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Re: HVAC intake ductwork freezing when it's cold (and sweating on drywall)
« Reply #5 on: January 21, 2015, 08:56:52 PM »
I think anything will help.  Those look like insulated flexible ducts, which have maybe R8 insulation.  If you fill the chase with insulation after making sure both ends of both layers (inner and outer flex duct/cover) are sealed well with foil tape.  Spray foam can also help in hard to reach or small places.

RichMoose

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Re: HVAC intake ductwork freezing when it's cold (and sweating on drywall)
« Reply #6 on: January 29, 2015, 10:53:31 AM »
How old is your house and ductwork system? Maybe my eyes are just not scaling right in the photo, but it seems your intake is quite a large diameter. Also, how far away from your furnace is the intake vent? When your furnace is running, what is the temperature of the hot air coming from your vents? How many cold air returns are located inside your house on each floor and what is the square footage?

With these answered, hopefully I can give some pointers.

Guses

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Re: HVAC intake ductwork freezing when it's cold (and sweating on drywall)
« Reply #7 on: January 30, 2015, 05:35:35 AM »
How old is your house and ductwork system? Maybe my eyes are just not scaling right in the photo, but it seems your intake is quite a large diameter. Also, how far away from your furnace is the intake vent? When your furnace is running, what is the temperature of the hot air coming from your vents? How many cold air returns are located inside your house on each floor and what is the square footage?

With these answered, hopefully I can give some pointers.

Hey there, our house is fairly recent 2007 construction. In the photo, there are actually two intakes I would say 4-5 inches in inner pipe diameter. Distance to furnace is approximately 15 feet.

I am not sure what temperature is coming out of the registers. It is hot enough that my cats like to sleep on the vents. Cold air return, 2 on main floor, 2 in basement. Bungalow, 1600 square feet footprint.

QajakBoy

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Re: HVAC intake ductwork freezing when it's cold (and sweating on drywall)
« Reply #8 on: January 30, 2015, 10:33:27 AM »
It sounds to me as if your fresh air ductwork needs a much better and expertly done insulation job. 
There are probably areas where the existing duct insulation may have internal voids or where it is heavily compressed and is severely reducing the insulation value, the surface toward your basement is getting too cold during the winter and existing indoor moisture is freezing/condensing.  Maybe with your extreme low winter temperatures you may need to encase the intake ductwork completely in spray foam.

Rube

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Re: HVAC intake ductwork freezing when it's cold (and sweating on drywall)
« Reply #9 on: January 30, 2015, 10:30:54 PM »
45-50% RH is a TON when it's that cold. At -30 I would stay <20%. A dry home is a happy home.

It's for sure an intake? I somehow in my mind reversed the locations of my intake and exhaust so when the exhaust was all wet it took me a couple of days to realize the inducer motor was full of water. Traced that back to plugged drain lines in the furnace.

RichMoose

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Re: HVAC intake ductwork freezing when it's cold (and sweating on drywall)
« Reply #10 on: January 31, 2015, 06:54:09 AM »
Ok, this is probably going to take a little work, but I think it's worth it. My guess is that one pipe is an intake and the other is an exhaust. Start by exposing both vents from the furnace to the exterior wall. The exterior wall is probably where your biggest problem lies. Many times, sloppy insulators or HVAC installers just force the ducts and intake together and forget to seal around the opening and "forget" to insulate this particular section of rim joist. If you have just a cheap intake, don't worry about it. Intake vents normally are fully open, the only thing between the opening and the duct should be some larger size mesh to keep mice out. Do not be tempted to cram fiberglass insulation in the opening to plug it. Your utility room needs this outside air to reduce CO buildup from combustion. As for the exhaust outlet (also called hood), it will often have a tin flap similar to a dryer vent. It may be worthwhile upgrading this outlet vent to a flap that is rimmed with foam insulation or some other form of one way closure. This will prevent cold back flow when your furnace or hot water tank is not burning. If you replace the exhaust outlet, make sure you  caulk around the opening in the wall on the exterior before pushing the outlet in the opening. Next, tape all joints in the ductwork with quality duct tape. Then, buy a bottle of that expanding spray foam and seal around the ducts to close the often oversized holes cut into the rim joists. It's not uncommon for sloppy HVAC guys to hack an 7" or bigger hole for a 5" outlet.

Then, using fiberglass batted insulation, insulate around the ducts to properly cover the rim joist with insulation. After this, wrap the ducts with insulation all the way to the furnace. Then, seal it up.

Don't cover this outlet that spews out cold air! If you are worried about the cold air getting into the rest of your house, build an insulated utility closet around your furnace and hot water tank.

Hope this helps.

 

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